Scientists inculcating a modern twist on a 50-year old technique have designed a hologram that could be supportive in making better smart glasses and enhanced anti-counterfeiting measures. It is a technique based on a metasurface, which is a design concept that offers fine control over numerous characteristics of light – the novel meta-hologram generates a powerful image at almost any wavelength of light, whether infrared or visible.
According to Antonio Ambrosio, a scholar in the Laboratory of Frederico Capasso at Harvard University, the novel meta-hologram is infinitely broadband. So because of this feature whatever light is generated, the users will always avail the same image with the equivalent quality of light.
It is also possible to design meta-holograms so that an image is visible only when it is illuminated or brightened by the light of a specific polarization. The polarization is an ability that can be useful in anti-counterfeiting seals on pharmaceuticals or money, as well as for swapping channels in optical telecommunications. The researchers define the details of their invention in the latest issue of few of the top-notch magazines.
The team depended on a hologram design, the binary detour-phase hologram that was first instigated in 1996. That unit utilized micro-apertures a little offset from each other – they detoured from a stringent periodic pattern to generate interference in the passage of light routing through them. As a result, there was an image created at a desired point in the space. The design was an essential step in the holography development, Ambrosio says, but it proved efficient only with light at a particular wavelength and passed on too little amount of the light. For such reasons, it was finally supplanted by other techniques, like liquid crystals and more.
The concept of the Harvard team was updated with the use of nanofabrication techniques to generate what they call ‘effective apertures’ out of items with little dimensions that are smaller than the wavelength of visible light. A series of nanofins was arranged wherein tiny silicon bars 85 nanometers wide, 1000 nm high and 350 nm long were placed. A group of ten nanofins was formed to create super pixels in which each one performed a specific function.
One major result of this experiment was that the individual pixels may work out with specific wavelengths, the hologram as a distinct unit does not care about the color of light that is shining through it. In one of the hologram versions, the nanofins in every pixel were inclined so that half of them can interact with the light spinning in a single direction and half with light in other direction. It enabled the hologram to showcase one out of two images, depending on the direction through which the light is passing. Such chiral holograms seem promising to be used for defeating counterfeiters.
Conclusion – While metasurfaces have been previously also used to generate holograms, most of them were reflective, confirms Ambrosio. This latest invention is trans missive, implying that it utilizes light shining through it. It is expected that it can project a holographic image, which may make it suitable to use in smart glasses which require for projecting an image in front of or into the wearer’s eyes.